On regenerative-overvoltage alarms (FANUC SRVO-044 HVAL, ABB 34400 DC-bus overvoltage and kin) the first suspect is often the braking resistor. This page writes out how we read, test and replace it.
A braking resistor is two numbers: nominal resistance (commonly tens of ohms depending on drive rating) and rated power (including short-time overload). Resistance sets braking current; power sets how much regenerative energy it can absorb. A replacement must match both: too high and braking is insufficient so the bus overvoltages; too low and overcurrent protection trips; too little power and the resistor overheats or burns.
Fixed order: power off, wait for the bus discharge indicator, verify dead - disconnect one end - measure resistance against nominal (drift beyond +/-10% counts as degraded) - megger the element to frame - visual check for cracks, discolouration and burnt leads. Resistors with a thermal switch get a continuity check on the switch too. The classic field misjudgement is "it conducts, so it is fine" - conducting but drifted still trips overvoltage.
When a regenerative-overvoltage alarm fires we chase three lines: the resistor itself (value / open), the brake chopper (acting? Fault codes?), and the duty (recent takt or load changes pushing regeneration past the original design). Swapping the resistor without the other two lines means the overvoltage returns.
Resistors run extremely hot - allow cooling before touching; keep the cooling path unobstructed at install; torque leads to manual and mark against loosening. Match the part number to the brand manual (different drive generations take different resistors); without a manual number, match on resistance/power/dimensions and record the substitution basis.
